An Islamic State group suicide bomber attacked a centre for Kurdish forces in the northern Syrian city of Raqa on Monday, killing four civilians and an anti-jihadist fighter, a monitor said.
The attacker blew himself up after entering the centre run by the Kurdish People's Protection Units (YPG), the Britain-based Syrian Observatory for Human Rights said.
"A suicide attacker wearing an explosives belt blew himself up inside a YPG centre after opening fire on a security checkpoint at its entrance," Observatory chief Rami Abdel Rahman said.
At least four civilians and a YPG fighter were killed in the attack, he added.
The YPG forms the backbone an alliance of Kurdish and Arab fighters called the Syrian Democratic Forces (SDF) battling to expel IS from its last redoubt in eastern Syria.
IS claimed responsibility for the attack they said targeted a "recruitment centre" for Kurdish forces.
"The attacker aimed at them with a machine gun then blew up his explosives vest in the middle of them," it said in a statement on the Telegram messaging app.
Backed by the US-led coalition, the SDF has in recent weeks advanced against IS fighters in their last holdout in the eastern province of Deir Ezzor near the Iraqi border.
The jihadists are clinging on to a handful of villages in the Euphrates River Valley, including Sousa and Baghouz.
They also retain a presence in the country's vast Badia desert.
IS swept across large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across areas under its control. But it has since lost most of that territory to various offensives.
The SDF ousted IS fighters from Raqa in 2017, more than three years after the jihadists overran the city and made it their de-facto Syrian capital.
Syria's war has killed more than 360,000 people and displaced millions more since it erupted in 2011 with the repression of anti-government protests.